In aircraft construction, a stringer or longer on or stiffener is a thin strip made of carbon fiber. Several strips are co-bonded on a cured skin in order to build a part of the aircraft (e.g. wings, horizontal tail plane, etc).
A very common type of stringers is that with a “T” shaped cross section defining a stringer web and a stringer foot.
Normally a manufacturing method for a “T” shaped stringer for an aircraft comprises a first step of hot-forming the laminates in order to achieve semi-stringers geometry with an “L” shaped cross section, and a second step of placing together two hot-formed semi-stringers to achieve a “T” shaped stringer. Then, the “T” shaped stringer is co-bonded on a cured skin with an adhesive line between them.
Such method comprises a third step, being a curing cycle. Usually, the curing tools are invar alloy angles of approximately 90° which are placed following the contour of the hot-formed “T” shaped stringer inside a vacuum bag during the curing cycle. It should be noted that invar angles are of 90° sharp when the skin on which the stringers are co-bonded is a flatskin.
When the skin is curved, the angle between the foot and the web of a stringer slightly varies, thus making it necessary to change the angle of the invar angle from 90°. In those cases, the “T” stringer does not fit perfectly the invar alloy angle geometry and it is possible to have deviations with respect to the stringer's theoretical position on the skin.
In the state of the art, when an invar alloy angle with a geometry having an angle different from 90° has to be used, the solution used is to place together the two “L” semi-stringers that form the “T” stringer at different heights. The problem of this solution is that the stringer feet and the surface where it has to be placed do not have the same geometry. When the stringer is placed on its position, there are free spaces between the stringer feet and the surface. Consequently, the union is a low quality union that can lead to future problems in the structure.
Another solution known from the prior art is to bend the feet of the stringer once it has been cured in order to adapt its geometry to the geometry of the surface where it has to be placed. The problem is that this bend of the stringer feet produces folds which are visible and which difficult the assembly of additional elements in the stringer.
The state of the art shows different approaches to T shaped stringer manufacturing process. Document EP 2 052 846 A1 reveals a method of forming a “T” shaped composite structure having a radius filler. Document US 2009/0107312 A1 discloses a method and apparatus to create bends in composite panels. Document U.S. Pat. No. 5,827,383 A shows a method of manufacturing a stiffener reinforced assembly.